J>LANT COVERING OF THE EARTH. 161 



Ways. We will consider some of its simpler effects, 

 paying particular attention to the influences which 

 most concern man. 



The most immediate effect produced by forests 

 is the improvement of the soil into which their 

 roots penetrate. Wherever trees succeed in finding 

 a foothold upon the surface of the earth, they pro- 

 ceed at once to make and to preserve a coating of 

 soil, which in the end may become fit for cultivation. 

 The roots penetrate downward into the crevices of 

 the rock, starting as slender filaments which, grow- 

 ing in size, wedge the stones apart and thus make 

 the beginnings of a soil. Into every cranny of the 

 disrupted stone, yet other roots find their way and 

 repeat the process of breaking. In this way in 

 the subsoil, the rock is fractured into bits, becomes 

 subjected to the dissolving action of the soil water, 

 and so affords food for plants. As long as the 

 rock remains in the condition of a solid mass, it 

 can yield but little plant-food. In that condition 

 there is only a small amount of surface for the 

 water to work upon. If we break a cubic foot of 

 that rock into bits a cubic inch in size, we multiply 

 the surface exposed to solution twelve-fold. If 

 these bits are in turn divided into cubes the 

 twelfth of an inch on a side, we again multiply 



